JP-A-2005-308380 conventionally discloses a refrigerant cycle device in which a branch portion for branching the flow of refrigerant is disposed on the upstream side of a nozzle portion of an ejector and on the downstream side of a radiator for radiating heat from the refrigerant discharged from a compressor. One of the refrigerants branched flows into the nozzle portion side, while the other refrigerant flows into a refrigerant suction port of the ejector.
In this refrigerant cycle device, a first evaporator (an outflow-side evaporator) is disposed on the downstream side of a diffuser portion of the ejector, and a throttle mechanism and a second evaporator (a suction-side evaporator) are disposed between the branch portion and the refrigerant suction port of the ejector. Each evaporator allows the refrigerant to exhibit a heat absorption effect.
The ejector which is applied to this kind of refrigerant cycle device expands the refrigerant isentropically at the nozzle portion, thereby recovering the loss of kinetic energy in expansion. This energy recovered (hereinafter referred to as “recovered energy”) is converted into pressure energy by the diffuser portion.
The refrigerant having a pressure increased by the diffuser portion is sucked into the compressor, thereby decreasing a driving power of the compressor, and thus achieving improvement of a cycle efficiency (COP). An ejector efficiency ηe indicative of energy conversion efficiency of the ejector is defined by the following formula 1:ηe=(1+Ge/Gnoz)×(ΔP/ρ)/Δi  (F1)where Ge is the flow rate of refrigerant sucked from the refrigerant suction port of the ejector, Gnoz is the flow rate of refrigerant passing through the nozzle portion of the ejector, ΔP is the amount of increase in pressure by the diffuser portion of the ejector, ρ is the density of the refrigerant sucked from the refrigerant suction port, and Δi is a difference in enthalpy between an inlet and an outlet of the nozzle portion.
Even when the dimension, shape, and the like of each component of the ejector are designed such that the ejector efficiency ηe becomes a desired value, as represented by the above-mentioned formula F1, the absolute amount of ΔP/ρ, which is an index representing the pressure energy converted by the diffuser portion, cannot be increased unless the absolute amount of the enthalpy difference Δi, which is an index representing the recovered energy by the nozzle portion, is increased.
That is, unless the absolute amount of the enthalpy difference Δi is increased, the absolute amount of increase in pressure ΔP cannot be increased at the predetermined ejector efficiency ηe. Therefore, it is impossible to enhance the effect of improvement in the cycle efficiency (COP) by the increase in pressure of the refrigerant sucked into the compressor.